1. Field of the Invention
The invention relates to a hydraulic positive displacement machine, especially a gear wheel machine, having a housing in which conveying elements, especially gear wheels, are supported by which hydraulic medium is taken in through an intake or suction opening and expelled under pressure via an outlet or pressure opening, and wherein the end faces of the gear wheels are sealed by at least one pressure plate. The hydraulic positive displacement machine further comprises a control device by which the pressure for loading the pressure plate into a sealing position can be adjusted. The invention also relates to such a control valve for controlling the hydraulic positive displacement machine.
2. Description of the Related Art
Gear wheel machines, for example, gear pumps, are used primarily in mobile hydraulics because they have a comparatively simple construction and allow relatively high pressures while being of minimal weight. A further advantage of gear wheel machines is that they can be employed in comparatively large rpm, temperature, and viscosity ranges.
In European patent document 0 445 529 B1 a gear pump is disclosed in which two gear wheels are rotatably supported in a housing. The displacement chambers are delimited by the meshing tooth flanks, the inner wall of the housing, and an axial plate or pressure plate. The latter is loaded on its backside by a hydraulic medium so that it contacts sealingly the end faces of the gear wheels. The pressure acting on the backside of the pressure plate can be adjusted by a control device in which a metering orifice embodied as a directional control valve and a three-port directional pressure regulator (pressure scale) cooperate. It is loaded in the sense of increasing the pressure at the backside by the pressure downstream of the metering orifice as well as a pressure spring and in the sense of a reduction of the pressure at the backside by the pressure upstream of the metering orifice. The pressure regulator (pressure scale) connects for this purpose a control connector connected to a pressure chamber on the backside of the pressure plate with the pressure outlet of the pump or with the tank, so that the sealing gap between the pressure plate and the end faces of the gear wheels is reduced or enlarged. For an enlarged sealing gap the hydraulic medium can flow directly from the high pressure side to the low pressure side so that the volumetric efficiency of the gear wheel machine can be reduced and, for example, the conveying volume or capacity of the gear pump can be adjusted as a function of the gap width. This means that by influencing the hydraulic medium pressure acting on the pressure plate an exact conveying flow regulation can be performed.
A problem of this configuration is that the pump housing must be comparatively complex since space for the directional control valve to the consumer and the pressure regulator (pressure scale) must be provided. Moreover, a separate pressure limitation valve must be provided in order to limit the maximum pressure.
In U.S. pat. No. 4, 014,630 a pump arrangement is disclosed in which a sealing plate is forced by a spring into the contact positioned on the conveying elements. The spring chamber is connected by a throttle bore to the displacement chamber of the pump arrangement so that the pressure plate is also hydraulically prestressed into its sealing position. The pressure acting on the pressure plate can again be adjusted by a control device via which the hydraulic medium can be returned to a tank. A disadvantage of this construction is that at all times hydraulic medium is removed through the throttle bore from the displacement chambers so that the volumetric efficiency is reduced. Moreover, the throughbore of the pressure plate can easily become clogged so that, correspondingly, a servicing of the pump arrangement is required.